Specifications

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4.6 SMALL COMPUTER SYSTEM INTERFACE (SCSI)
The board's SCSI bus interface is completely controlled by an AIC-6250 SCSI protocol controller
device. The AIC-6250 contains a complete ANSI X3.131 compatible SCSI bus interface,
including 48 mA bus drivers, 8-bit data FIFO buffer, and hardware support for bus arbitration,
REQ/ACK handshaking, asynchronous and synchronous SCSI data transfer modes, and all
required SCSI Initiator and Target functions. (Although the board's SCSI hardware has the ability
to support for SCSI Target functions, disconnect/reselect, and synchronous data transfers, these
functions are not implemented in the current SCSI/BIOS.)
Please refer to the AIC-6250 technical manual for additional technical details on the SCSI
controller device:
AIC-6250 Technical Manual
Adaptec, Inc.
691 S. Milpitas Blvd.
Milpitas, CA 95035
For more information on the SCSI interface in general, refer to the American National Standards
Institute document, ANSI X3.131-1986 available from:
American National Standards Institute
1430 Broadway
New York, NY 10018
When the SCSI option is present, the AIC-6250's DMA request output signal connects to DMA
channel 7. This channel provides a 16-bit DMA data path to system RAM memory on the Little
Board/286. Note that, as in the standard AT architecture, this DMA channel cannot transfer data
beginning on odd-byte boundaries. When a transfer to an odd-byte aligned buffer is requested by
software utilizing ROM-BIOS functions, the block of data is first transferred by the ROM-BIOS to
a temporary buffer having an even-byte buffer, and then shifted by a memory block move to the
desired odd-byte aligned destination. Therefore, maximum SCSI transfer speed can be realized if
buffers are aligned to even-byte boundaries. Otherwise there is approximately 5% degradation in
throughput.
A jumper option (W2) allows the interrupt output of the AIC-6250 to be connected to the IRQ15
system interrupt. Though not currently used, IRQ15 is reserved for use by the board's ROM-BIOS
and SCSI support software.
Socketed 220/330 ohm bus termination is provided on the board. In addition, a jumper pair can be
shorted to provide power to the SCSI "TERMPWR" signal, which is required if external
termination networks are used. As recommended by the SCSI specification, a Shottky diode on
the board limits the flow of current between the board's +5V power and the SCSI TERMPWR line
to be away from the board. This prevents system damage from occurring in the event other
devices on the SCSI bus are also supplying power to the TERMPWR line.
Note that you can eliminate the board's SCSI hardware function by removing the AIC-6250 from
its socket on the board. When the port is disabled in this manner, its I/O port addresses and
system interrupt and DMA support become available on the AT Expansion Bus for other devices
in the system.